Homopolymers and copolymers of ethylene with higher alpha-olefins such as 1-butene, 1-pentene, 1-hexene or 1-octene may be prepared, for example, by polymerization in the presence of supported chromium compounds, known as Phillips catalysts. U.S. Pat. No. 6,699,947 describes supported chromium catalysts and their preparation, whereby a support material is treated in suspension with a chromium salt solution and subsequently, after removing the solvent, is calcinated in an oxygen-containing atmosphere at temperatures above 300° C. Catalyst having a mean particle size of less than 100 μm is obtained by grinding or comminuting either the support or the catalyst at any stage of preparation.
Suspension polymerization processes have been known for a long time already. A process in which the polymerization is carried out in a loop reactor is described in WO2005/028098 and is particularly useful for the polymerization of ethylene and of ethylene together with other olefinic comonomers. In loop reactors, the polymerization mix is pumped continuously through the cyclic reactor tube. Such circulation of the polymerization mix helps to approach an optimum homogeneization of the reaction mix, it improves the catalyst distribution and also prevents sedimentation of the suspended polymer.
Also U.S. Pat. No. 6,239,235 describes a polymerization process in a loop reactor, in which an increase in the average proportion of solids of up to 53% wt in the reactor is achieved by means of a continuous discharge system.
Many ethylene polymers and co-polymers are sold commercially as powder grades. For efficient logistics and increased line speed during the processing of such powder grades, a high bulk density of the products is required to maintain economic conditions in the industrial production.
In WO03/054035 it is described the use of a mixture of two ZN catalysts comprising a titanium and an electron donor compound supported on magnesium chloride, having different average particle size and/or different porosity in order to produce in a loop reactor, propylene polymers having either increased bulk density or, for the same bulk density, increased porosity. Due to the fact that the disclosure is focused on the preparation of propylene polymer, it does not address the problem of the generation of fine particles of polymer when producing polymer particles having smaller average size.
In fact, it is well known that smaller catalyst particles produce smaller polymer particles with higher bulk density. However, such products show unfortunately a totally unacceptable high amount of fines, whereby more than 1.5% of the particles have a particle diameter of less than 125 μm. Particularly in ethylene polymerization, such high amount of fines implicates the risk of dust explosions due to electrostatic charging and/or conveying problems and, thus, is the reason for big technical problems during processing. As smaller polymer particles are, as more they tend to electrostatic charging. Such charged particles increase the risk of operability problems during polymerization by wall sheeting and reactor fouling, as may be recognized as increased power input of the reactor pump.